1. Field of the Invention
The present invention relates to a semiconductor manufacturing process and in particular to a method for forming deep trenches in dynamic random access memory (DRAM).
2. Description of the Related Art
DRAM is capable of reading and writing information. Each DRAM cell requires only one transistor and one capacitor; therefore, it is easy to achieve higher integration and is broadly applicable to computers and electronic equipment. A trench capacitor, formed in the semiconductor silicon substrate, is one of the most commonly used capacitors. The surface area of the trench capacitor is increased by deepening the trench capacitor in the semiconductor silicon substrate thereby increasing capacitance. A chip with trench capacitors can be separated into a memory cell array area to store data and a decoupling capacitor area to filter noise.
In the conventional trench capacitor fabrication method, a plurality of trenches is formed in a semiconductor silicon substrate. The silicon semiconductor substrate with trenches covers an As-doped silicon oxide layer. The silicon oxide layer is patterned with lower electrode patterns by coating and baking a photoresist. The photoresist can flow into the trenches in the baking step. After baking, the hardened photoresist is removed by dry etching until the upper surface of the photoresist is lower than that of the semiconductor silicon substrate by a predetermined distance. The exposed silicon oxide layer is removed using the photoresist as a mask. Then, the doped As ions in the silicon oxide layer are driven into the semiconductor silicon substrate to form a conductive layer as a lower electrode of the trench capacitor. The capacitance of the trench capacitor is related to the surface area of the lower electrode, and determined by the area of the silicon oxide layer covering the trench. The area of the silicon oxide layer covering the trench is controlled by the distance between the upper surface of the photoresist and that of the semiconductor silicon substrate. However, the distance is difficult to control.
Adhesion between the photoresist and the silicon oxide layer is too weak to drive the photoresist flow into the trench after spin coating. During the baking of the photoresist, the photoresist can flow into the trenches. Nevertheless, the densities of the trenches in the memory cell array area and in the decoupling capacitor area are different. A mass of the photoresist flows into the trenches in the higher density area (the memory cell array area) so that the surface of the hardened photoresist is lower. A small quantity of the photoresist flows into the trenches in the lower density area (the decoupling capacitor area) so that the surface of the hardened resist is higher. Therefore, there is a height difference between the surfaces of the hardened resist in different areas.
Moreover, the difference between the distances between the upper surface of the photoresist and that of the semiconductor silicon substrate in different trenches exists after etching the photoresist. Under the 0.175 xcexcm design requirement, for example, the above-mentioned difference reaches 8200 xc3x85. In order to prevent the lower electrode in the lower density area (the decoupling capacitor area) and the buried strap (or the so-called ion doped band) from subsequently forming in the top of the semiconductor silicon substrate by shorting, the lower electrode formed in the memory cell array area has a smaller surface area, which is detrimental to storage performance. In order to increase the surface area of the lower electrode in the memory cell array area, the breakdown voltage between the lower electrode in the decoupling capacitor area and the buried strap must be reduced, but the reduction may cause a short circuit. Therefore, etching the photoresist is difficult and it is possible that the entire process may fail.
In order to solve the photoresist layer uniformity problem, a method of improving uniformity is provided in which the substrate is modified to enhance the adhesion between the substrate and the photoresist layer. The modification of the substrate comprises an oxygen plasma treatment, a wet treatment with a mixed solution of H2SO4 and H2O2, or a wet treatment with a mixed solution of NH4OH and H2O2. After this modification, the difference between the upper surfaces of the recessed photoresist layer in each of the trenches is controlled and reduced by 3000-4000 xc3x85.
When the design requirement reaches 0.11 xcexcm, however, the difference in non-uniformity between the recessed photoresist layers in each of the trenches can reach 7000-8000 xc3x85. Therefore, a method of improving uniformity in the photoresist layer is required.
Accordingly, an object of the invention is to provide a method for forming bottle-shaped trenches, such that all trenches have the same depth regardless of whether the trenches are located in a dense area or in an isolated area.
It is another object of the present invention to provide a method for forming bottle-shaped trenches to prevent the capacitors in the decoupling capacitor area from invalidation and prevent capacitance reduction in the memory cell array area.
It is still another object of the present invention to provide a method for forming bottle-shaped trenches which are suitable for fabricating trench capacitors to increase breakdown voltage between the lower electrode and the dopants and enhance the reliability of the trench capacitors.
It is a further object of the present invention to provide a method for forming bottle-shaped trenches to simplify the manufacturing process and increase process windows.
One feature of the present invention is the use of polysilicon as a shield layer to replace the photoresist in the conventional bottle-shaped trenches manufacturing technology to avoid non-uniformity of photoresist recesses which lead to different depths of each of the lower electrodes.
Another feature of the present invention is the formation of the protective layer comprising a nitride on the sidewall of the upper portion of the trenches by deposition, such that the thickness of the protective layer can be effectively controlled. Compared to the prior art of the formation of the nitride protective layer by nitridation, the thick protective layer of the present invention can sufficiently protect the upper portion of the trenches from damaging during wet etching for forming bottle-shaped trenches. However, the conventional nitride protective layer is not thick enough to provide sufficient protection during wet etching, such that the bottom area of the bottle-shaped trench is limited.
To achieve the above objects, one aspect of the present invention provides a method for forming bottle-shaped trenches. First, a substrate is provided. Next, a hard mask with openings is formed on the substrate. The substrate is etched through the openings to form trenches with an upper portion and a lower portion. An isolated layer is formed conformally on the hard mask and in the trenches. A shield layer is formed in the lower portion of the trenches. A part of the insulating layer, which is not covered by the shield layer, is then removed. A protective layer is formed on the upper portion of the trenches. The shield layer and the isolated layer are removed. Finally, the substrate of the lower part of the trenches is wet etched using the protective layer as a mask so as to form bottle-shaped trenches.
According to the present invention, the protective layer comprises dopants driven into the substrate surrounded the protective layer by thermal treatment. The material of the protective layer comprises a nitride. The formation of the protective layer comprises the steps as following. First, the protective layer is conformally formed on the sidewall and the bottom of the trenches by chemical vapor deposition (CVD). Then, parts of the protective layer on the bottom of the trenches are removed to leave parts of the protective layer on the sidewall of the trenches.
According to the present invention, the shield layer comprises polysilicon. The formation of the shield layer in the lower portion of the trenches comprises the steps as following. First, the shield layer is formed to fill the trenches. Then, parts of the shield layer in the upper portion of the trenches are removed so as to leave parts of the shield layer in the lower portion of the trenches.
A detailed description is given in the following embodiments with reference to the accompanying drawings.